Does water affect line tension?

I’ve heard conflicting theories about what water (from rain, from wet feet, etc) does to standing tension of a slackline. Mostly these theories look at nylon lines, which are hydrophilic (meaning they easily soak up water). Polyester is hydrophobic, so there is less concern there, as it’s difficult to saturate polyester with water. We know that water probably doesn’t help break strength, so you should avoid getting any very tight line wet, but it would be nice to know what the water will do to the tension. Does it rise? Does it fall? Most people, in my experience, seem to think it rises. My personal experience, from walking a lot of dry waterlines that eventually get wet, tells me that it falls. Let’s do a test!

Cliff notes: if you don’t want to watch the video, the tension does indeed fall, from 1100lbs to 925lbs. This was a 56.5′ line with a standing (read: settled) tension of 1100lbs, and it experienced a drop of almost 20% of its tension. Line length, tension, the amount of water, the humidity, and probably some other factors will all affect how much tension falls, but I think this experiment strongly suggests tension will likely fall in most if not all cases.

1 Comment

Awesome, but I REALLY would like to see what happens when it dries under tension. Does the tension return to it’s original value? Does tension rise as the line dries, but settle at a lower value, or does it even end up higher than it was originally? Just going off of feel, it definitely gets tighter (than it’s wet state) as it dries. But trying to quantify that with your feet instead of a dyno is imprecise to say the least and I really can’t tell if it tightens up even more than it was originally, but it does feel that way.

My line often feels different for a dry rig or two after being used as a water line, which makes me wonder if the process of getting wet and drying repeatedly does cause some temporary change to the fibers.

I know it’d be a much longer test, but if you ever have the chance to repeat this, leave it rigged in the sun to dry out and see what happens, pretty please? Feel free to bring your rig down south if you don’t have enough sun to dry the line out!

Adam’s Comments:

Yeah, haha, as I was doing this test I was thinking I should have left time to let it dry. I will repeat this in the summer or late spring. My intuition says it tensions back up, although I think others (who believe water is only acting as a lubricant here) disagree with me. So we will see!